Fuel elements for nuclear reactors



y 1964 M. c. HARTNELL-BEAVIS 3,133,000

FUEL ELEMENTS FOR NUCLEAR REACTORS 5 Sheets-Sheet 1 Filed July 1 1959FigH 1 Fig.10

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M. C. HARTNELLBEAVIS FUEL ELEMENTS FOR NUCLEAR REACTORS May 12, 1964Filed July 1,

May 12, 1964 M. c. HARTNELL-BEAVIS 3,133,000

FUEL ELEMENTS FIOR NUCLEAR REACTORS 5 Sheets-Sheet 3 Filed July 1, 195942945;, 42 444 H TDRMe- S M. c. HARTNELL-BEAVIS 3,133,000

FUEL ELEMENTS FOR NUCLEAR REACTORS May 12, 1964 5 Sheets-Sheet 4 FiledJuly 1, 1959 INVENTOK MCWEL L TMeu-Bsan:

T R EYS May 12, 1964 M. c. HARTNELL-BEAVIS 3,133,000

FUEL ELEMENTS FOR NUCLEAR REACTORS Filed July 1, 1959 5 Sheets-Sheet 5Fig. \3

ii l ICW G L Ahpwac' Zgnvg BY Liam, 24a,-

3,133,000 Patented May 12, 1964 i 3 133000 FUEL ELEMENTS EoR NUCLEARREACTORS Michael Carl .Hartnell-Beavis, Longfield, Kent,- England,

assignor to The General Electric Company Limited,

London, England Filed July 1, 1959, Ser. No. 824,248

Claims priority, application Great Britain July 2, 1958 Q i a 9 Claims.(Cl. 176-48) This invention relates to fuel elements for nuclear reactorsand more particularly to fuel elements for use in nuclear reactorsof the kind in which fuel elements are received within channels throughwhich fluid heattrans:

fer medium is arranged to flow to extract heat from the elements duringoperation of the reactor. Thus thereactor may be of the heterogeneoustype having a core of said moderating material, such as graphite, formedwith fuel 'channels or alternatively it may be a reactor of thetype'using a liquid moderator in which the fuel channels arepressuretubes within the moderator.

With the development of fuels enriched in fissile isotopes it has beenpossible to avoid the use of fuels which .suflerfrom appreciabledeformation on passing through the uranium metal transition temperaturesas they heat up, and to increase the fuel heat rating not only by perof,say, to

mitting higher working temperatures, but also by increasing the surfacearea to mass ratio of the elements. This has been achieved in the pastby forming the fuel ele m'ents as :bundles of small diameter tubes orrods or as thin plates orthe like; uranium oxide and uranium carbide areparticularly advantageous fuels for this purpose since in addition theypossess good long term irradiation properties, It is however desirableto sheath these fuels, for reasons which are well-known, in animpermeable 1 material, suchas stainless steel, beryllium or the like,

which has the requisitehigh temperature properties consistent with lowneutron absorption characteristics and compatibility considerations;

. '1 :These high surface'area elements'have beenmounted longitudinallywith respect to the fuel'channels, the cool' ant flowing longitudinallyover the elements. "Moreover,

in order adequately to support the elements, not only I have endmountings to be provided but intermediate ones,

too; withthe result that additional parasitic neutron absorption andhighpressure losses in the coolant flow are 1 caused, notwithstanding highercosts involved in the dif- *ficult problem of manufacturing these.complex supports.

It is object of the present invention to obviate the ,7 use of suchsupports while at the same time providing a means for'obtaining goodheat transfer conditions with A low pressure losses.

In' accordancewith the invention, a fuel element 'a'r rangement for. anuclear reactor of the kind specified comprises acarrier adapted forinsertion in a said channel; said carrier being formed with or providingat least one duct adapted for flow therethrough of said fluid heattransfer medium, and a plurality of fuel members arranged across'saidoreach said 'duct at intervals spaced in the direction of fluid flowthrough the duct or ducts. This carrier is preferably of moderatingmaterial, such as graphite. J p Y The fuel members may 'be of anydesired section, but

' are preferably of a section giving a low resistance to flow, such asstreamline, oval,-ellip'tical or lenticular sec: 7 tion, all hereinaftergenerically referred tov as oval.

They may alternatively beof circular plate or other convenient shape. TTheymay conveniently be supported by their' ends in said carrier andthey may be assembled bymeans'ofspacersarranged to support the ends ofsaid members:

a The said [carrier may be composed of at least two porin accordancecesses in said'carrie'r (or said fuel members).

In one preferred construction, the fuel members are arranged in a seriesof rows and it may be convenient and beneficial, then, toarran'gethatthe members are at an inclination to aline paralleltothe'longitudinal axis of the duct in which they are supported. With thelatter arrangement, alternate fuel members in someat least of said rowsmaybe inclined at substantially equalbut opposite directions to theirneighbours. Certain benefits are discussed below and it would appearthat greater benefit will accrue when the inclination is in therangethan when it is greater than 45. It will usually benecessary to includeend spacers within each said fuel member to prevent overheating of theends of the members, and such spacers may be formed ofmoderatingmaterial. Alternatively' such spacers may be forrnedof refractorymaterial, such as of magnesium oXide, having low neutron absorptioncharacteristics; they may also be porous with the object of providingvoidage for the accommodation and retentionof fission product gases.'

It--may be desirable to provide means on said carrier to orientate saidcarrier, and therefore tlie'fuel members,

within asaid channel of -a reactor core and it may then arfangecooperating means in said chana parallel to the longitudinalaxis of thecarrier.

In -a preferred construction, the fuel members areso arranged that, inthe general directionof fiuid'flow, one fuelrnember of onerow is alignedwith a space between fuel members'of thenextroW; V In anotherconstruction, some of the lrows of fuel members may be'at an angle toother rowsfand the rows may even be at right angles to each other. VEspecially if the fuel members are of streamline section, the turbulentdrag 'on I the fluid medium passing through-the duct will' be kept to aminimum and in any event, losses of the prior arrangements abovedescribed,

introduced by the endsuppoits of the longitudinallyjdisposed elements,are completelyobviated-since there need be no support in the flow pathof the fluid me'dium.

F Other advantages of a fuelelement arrangement in ac cordance with theinvention, plate-type elements are:

even over the conventional (a) It is possibleito extrude oi pressceramic or cermet fuel. material with' a desirable eross-section-and toexthat there are no joints or welds along the length of the fuelmembers; i l

. -(b) Theisheaths can have a continuous positive radius of'curvatureround a section, thus minimising distortion and-separation ,of thesheath wall from the fuel material trude or draw sheathing material ofthe required shape, so

over a large area, such as might-occur on the-sides ofa plate-type ofelement due to the'local distortion of the fuel element material orcollection of. fissionpro'duction gases. 2 I 1 It will be understoodthat a fuel element arrangement with the invention may-"be. a compositestructure consisting of fuel members placed across-a duct in a carrierwhich may, hen beinserted as. a unit into a said fuelchannel in amoderating core.

, Injorder that the invention may be better understood;-

fuel element arrangements will now be described by way tionalrepresentation on the line AA of FIGURE 1; and

FIGURE 3 shows a part-sectional representation on the line BB in FIGURE2. 7

Another arrangement is illustrated in FIGURES 4 to 7, of which FIGURE 4shows an elevation in part section on the line CC of FIGURE 5, of thisarrangement, FIGURE 5 shows an end View of the arrangement; FIG- URE 6shows a part-sectional elevation of'one form of fuel member for thisarrangement and FIGURE 7 a sec tion on the line DD in FIGURE 6.

FIGURE 8 shows a sectional view of an alternative fuel member of whichthe sheathing is graphite.

FIGURE 9,illustrates a construction of fuel element arrangement whichis' a modification of that shown in FIGURES 4 and 5.

FIGURES l0 and 11 are representations in directions at right angles toeach other, illustrating a more elaborate form of fuel member. I

Whilst the arrangement shown "in FIGURES 1 to 3 is intended for use in areactor where the fuel channels are substantially vertical, and that ofFIGURES 4 to 7 4 bers prevent excessive turbulent drag in the wake ofthese members.

It will be appreciated that the above example may be considerablymodified within the scope of the invention. Thus more elaborate shapesof fuel members may be used, as well as more simple, and it may be foundconvenient to provide a duct of cross-section other than the square onedescribed. It is also envisaged that the carrier could be of skeletonformation.

In the arrangement illustrated in FIGURES 4 to 7, the carrier, in theform of a graphite sleeve, is formed in two halves. The flow duct inthis case is irregular in shape to allow of fuel members of threedifferent lengths, as shown in FIGURE 5. r

The joints between the halves of the sleeve are at and 21, and graphitedowels 22, 23 are spaced along each joint to locate the halvesrelatively to each other. H-links 24 .of graphite are arranged insuitableslots at spaced intervals along the lengths of the joints tokeep the two halves together; these links are locked in position bylongitudinal graphite pins 25 which, for convenience of length areinserted from either end along holes bored in the sleeve walls. Thesepins in turn are retained in posi where they are substantiallyhorizontal, it will be evi- V dent that little, if any, modificationwould be required to render the respective arrangements suitable forhorizontal or vertical operation. 7

Referring now to FIGURES l, 2 and 3, the arrangement comprises a numberof rows of fuel members, in two groups of columns, the end members ofrows of one such group being labelled 2a and those of the rows of theother group, 2b, the members of group 2a being arranged above spacesbetween the members of group 2b. The members are, as indicated,substantially elliptical in section with the major axis of the ellipsesubstantially in the direction of the fluid flow. They comprise a charge3 of fuel (see FIG. 3) encased within a sheath 4 of anysuitablematerial. The sheath has end caps 5 of similar material andgraphite spacers 6 are included at each end to prevent overheating ofthe ends of the fuel members. Sheathing is completed in such a way thatas much voidage as possible is left for the accumulation and retentionof fission product gases. I

tion by steel clips which engage recesses 26 cut into the walls of theseholes.

One end 27 ofthe sleeve is formed as a spigot which engages a locatingbore, similar to that shown at 28, in an adjoining sleeve to maintaincontinuity of the fluid flow duct in successive sleeves when in a fuelchannel. Locating arrangements (not visible) are provided to locateadjoining sleeves relatively to'each other, so as to align the fuelmembers with respect to each other. a .In this design the fuel members29 are arranged in rows of six as indicated in FIGURE 5 and there arefifteen rows along the length of the sleeve.

The fuel members are shown in greater detail in FIG- URES 6 and 7. Thestainless steel sheathing of each member is of elliptical (oval) sectionand'the fuel proper is in the form of pellets 30, of which there are,say, eight per member, of uranium dioxide, which are formed'to shape andassembled into the sheaths-so that thermal contact may be attainedbetween fuel and sheath. A magnesium oxide end spacer 31 is provided ateach end of Y the member to protect the end caps 32 from the high Thefuel members are supported at their ends in slots 7 formed in graphitespacers 8 which are assembled in sets of spaced grooves formed inoposite walls of an approximately square duct 9 in a carrier 1 ofgraphite or other suitable. material. The standard spacer 8a issymmetrical, but at the top and bottom of each line of members, spacersspacers 8b or 8c are necessary to complete the columns, which are heldin place by end pieces 10. These end piecesare keyed at either side intothe slots llat'the end of the carrier 1 and are retained by pegs 12.Although only one end of the arrangement is shown in thedrawings, it isto be understood thatsimilar construc ment is shown in position in sucha channel; in FIGURE l; and it willbe seen that the carrier. is recessedat diametrically opposite regions for engagement by keys 14 which areprovided in the fuel channels to orientate the arrangement.

In operation, the cooling fluid flows through the duct 9 lengthwise ofthe arrangement and it is seen that there is little resistance totheflow since, except for the fuel members themselves, there is noobstruction in the duct, and in addition the streamlined, i.e., oval,shape of the membe stamped integrally with the end caps. of thesespigots is to retain the fuel members in position temperatures. of thefuel in operation. The end caps are welded to the sheathing and carryspigots 33, in the form of stampings, or they could be solid rods orcould even The, function in the flow duct and for this purpose theyengage holes in the wall of the graphite sleeve, as shown in FIGURE 4.

This particular arrangement is intended for high temperature operation,and, to provide for a slight reduction of the central temperature of thepellets, these are provided with holes 34 and 35 as indicated in FIGURES6 and 7. The presence of these holes will also provide space for thecollection of fission product gases.

There is no necessity for the sheathing of the fuel members to bemetallic. In FIGURE 8, a fuel member is i1- lustrated which is made withan impermeable graphite sheath 36. Types of impermeable graphite are nowavailable which have very good mechanical properties and this materialhas a good performance at higher temperatures than at. which otherknown-sheathing materials can be used.

i In order to obtain good thermal transfer, the fuel would be in theform of rods or pellets 37, loaded with a filling of-a spongy? graphite.38 into the outer sheath.

The complete assembly of fuel members in a graphite sleeve is preferablyformed with means which can be engaged for charging and discharging itinto or from a fuel channel in a reactor core and cooling fluid flowinginto a said channel will then traverse the duct in the sleeve with theminimum of resistance from the fuel members, in virtue of theirstreamline form.

7 instead of the spigot and prevented.

I URES' l and 11.

the length of the fuel member (FIG.

I i A modification of this arrangement is illustrated in partin FIGURE 9and will be clear when studied in conjunction with FIGURES 4 and 5. Herethe rows of fuel mel'nbers are arranged at an angle to the transverseaxis ofthe sleeve. Altern'ate. members in each row-are inclined inopposite directions, so that three members in eachrow are inclined ineach direction. In the illustration, alternate rows of fuel membershavebeen omitted for clarityjand. it will be understood "that, for thesame loading, substantially the same number of fuel members will berequired as for the arrangement of FIGURES 4 and 5. I

1 The .optimum'angles of inclination will be a matterfor experimentationin each particular case,

7 depending upon loading, flow rates,'fluid temperatures and so on. Itmay also be found necessary to apply the groove and spacer technique ofthe construction shown in FIGURES 1 to 3,

, hole arrangement, to position the inclined'fuel members. j i 1 I Oneadvantage of the arrangement of FIGURE 9is that, if the fuel members areclose enough to each other, it has been found that the streaming offluid along one "member in operation is aifected by the proximity 'ofthe other crossing member. The effect of this phenomenon is that anypulsatingmovement o'fth'e fluid, which tends to' be set up as it flowspast the 'fuel'm'embers, appears to be broken up; in this way it isprobable that vibration which might otherwise tend tob' setup in thefuel element, is

Accordingto another feature of the invention the sheathing offuelmemb'rs'arran'ged' for cross flow of fluid is provided with fins or' thelike 'to' enhance heat transfer characteristics of the fuel members.

, One form of finned fuel member is illustrated in FIG- The fuelmembercomprises a charge ofrfuel such as has been described above. Thesheathing is formed from a suitable material, possibly impermeablegraphite, and the member and-fins are shown as being of lenticularstreamline section. The sheathing has a number of fins, as shown, whichare in good thermal con tact with its surface and possibly formedintegrally therewith. The fins are preferably of similar material tothat bf the sheath, and may be spaced equally or the spacing may begraded along part orthe whole of the length of the sheath. The fins neednot essentially be of similar outline to the sheath'in side view, asshown in FIGURE 11; nor

, need the fins be shaped in section as indicated in FIGURE 10, althoughthis is desirable from the point of better heat transfer to the fluidflowing in the system, flow being ar- I ranged to be in the direction ofthe arrow in FIGURE'll.

, With different fuels and cladding materials it may be more or lessadvantageous toprovide fins. Thus, in general, with-graphite cladding itwill definitely be an advantage to provide them. It will be a matter forcon sideration, however, in the cases of beryllium and of socalled .Magnox-' cladding materials; but with stainless V steel,-'-finning will notbe a proposition, if only on'account of its low thermal conductivity. I

Since the "temperature of the fuel in the fuel member vary with distancefrom the wall of the channel in'which the fuel element arrangement ispositioned, it may be desirable to relate the characteristics of thefins on the fuel members to the positions of the fins along the lengthof the fuel member. Thus they may be arranged to be smaller nearer themiddle of the fuel member than near the ends (FIG. '12), so as to ensuregreater heat transfer from the ends which will tend to run hotter thanthe middle of the fuel member. Alternatively or additionally the spacingof the fins may be varied along For similar reasons, it may be desirableto arrange that the fuel members themselves, irrespective of whethervthey are finned or not, are spaced at different intervals across-theduct in which they are positioned.

. It is known that increasing the pressure of coolant will provide ameans of achieving'a greater heat transfer on the output side of agas-cooledreactor, and'pressures ofup to 60 atmospheres have in factbeenproposed. The increase of pressure'- drop at these} atmospheres doeshowever lead to difiicultyin supporting fuel elements of the knownlongitudinal' flow type. The form of element arrangement in accordancewith the invention is however ideally suited to" the high pressure typeof reactor and support may readily be made adequate, even at pressuresof the order of 100 atmospheres.

According to anotheraspect of the invention, therefore, a high outputgas-cooled reactor is adapted for operation at at least 80atmospherespre'ssure of coolant.

Owing to l elimitations of heat-transfer in longitudinal flow over fuelelements'of known type, gas-cooled graphite-moderated cores with outputshigher than, say 100 megawatts (thermaD-require pressure vessels of suchlarge size that they cannot be fabricated as" a single unit under :thestrictly controlled conditions of a works since they cannot then betransported. It has been considered that they have to bewelded on site;but this requires a severe limitation on thickne'ssof wall of thepressure vessels and hence far less than those possible with shopweldedvessels. However an appreciable increase in per;

formance can be obtained with crossfiow element ar-" Although uraniumoxide and funanium carbide arestated to be particularly advantageousfuels we do not wish it to be taken that element arrangements comprisingfuels which aresubject to the disadvantage that they passthroughtransition temperatures; if the tempenature is allowed to attainsuch levels, are essentially excluded from the scope of the" presentinvention. Even if the operating temperatures of the fuel elements arepermitted to rise only to within a safe margin of the first transitiontemperature of uranium metal, so that uranium metal may in fact be used,a fuel arrangement in accordance with the invention permits of thedesign and operation of a reactor-having far greater output than knownheterogeneous reactors of like size. Other metallic fuel materials maybe similarly used, in spite of a limitation on upper temperature of use,and yet give rise to possibilities of enhanced output. Especially by theprovision of fins on fuel member sheaths referred to above, it isestimated that for the same coolant pumping power to thermal inputratio, the thermal output per channel can be in creased over that forlongitudinal flow about five times for a channel system less than halfthe length by the use of the cross-flow type of element arrangement inaccordance with the invention.

I claim:

1. In a nuclear reactor having an elongated channel in which fuelmembers are arranged to be disposed and through which channel a fluid=heat tnansfer medium is arranged to flow in a direction parallel to thelongitudinal axis of the channel to extract heat from such membersduring operation of the reactor: a fuel arrangement comprising v v (a)an elongated carrier adapted to be inserted in said channel,

(b) said carrier including means providing an internal duct within saidcarrier -(i) which duct is defined by open ends and by longitudinal axesof the channel and of the carsectional dimensions which both are smallin relation to the Width and length of the duct and thereby providespace for the flow of the heat transfer medium in a direction generallyparallel to the longitudinal axis of the duct,

, (iv) said fuel members being mutually spaced apart and being locatedat intervals along the longitudinal axis of the duct,

(v) each of said fuel members being of oval transverse cross-sectionWitht-he major dimension thereof extending in a direction parallel tothe longitudinal axis of the duct. 1 a

2. A fuel arrangement as set forth inclaim 1 wherein the carrier iscomposed of two sections, the joints between which extend in a directionparallel to the longitudinal axis of the carrier. I

' 3. A fuel arrangement as set forth in claim 1 wherein the fuel membersare arranged in a series of-rows each row of, which extends transverselyacross the duct, the series of rows extending relative to one another ina direction parallel to the longitudinal axis of the duct, said'fruelmembers providing clear spaces therebetween extending in 'a directionparallel to the longitudinal axis of the duct.

4. A fuel'arrangement'as set forth in claim 1 wherein the longitudinalaxes of at least some of the fuel members are inclined to thelongitudinal axis of the duct.

5. A fuel arrangement as set forth in claim 4 wherein the fuel membersare arranged in a series of rows each row of which extends tnansvers'elyacross'the duct, the series of rows extending relative to one another ina direction parallel to the longitudinal axis of the duct and whereinalternate .fuel members in at least some of the rows arev inclined atsubstantially equal but opposite directions to adjacent fuel members.

6. A fuel arnangement as set forth in claim 1 wherein each of the fuelmembers includes a sheath and wherein at least some of the sheathshavefins lying in planes parallel to the longitudinal am's of the duct.

7. A fuel arrangement as set forth in claim 6 wherein thecharacteristics of the fins vary with respect to the positions of thefins along the longitudinal axis of a fuel member having the same. a l

8. A fuel arnangement as set forth in claim 7 wherein the fins arevariably" spaced along the longitudinal axis of the fuel member.

9; A fuel arrangement as set forthin claim 7 wherein the size of thefinsvary according to the distance of the fins from the adjacent end ofthe fuel member.

References Citedin the file of this patent UNITED STATES PATENTS OTHERREFERENCES AEC Document NAA-SR-1936, September 1, 1957, in

particular page 21.

1. IN A NUCLEAR REACTOR HAVING AN ELONGATED CHANNEL IN WHICH FUEL MEMBRSARE ARRANGED TO BE DISPOSED AND THROUGH WHICH CHANNEL A FLUID HEATTRANSFER MEDIUM IS ARRANGED TO FLOW IN A DIRECTION PARALLEL TO THELONGITUDINAL AXIS OF THE CHANNEL TO EXTRACT HEAT FROM SUCH MEMBERSDURING OPERATION OF THE REACTOR: A FUEL ARRANGEMENT COMPRISING (A) ANELONGATED CARRIER ADAPTED TO BE INSERTED IN SAID CHANNEL, (B) SAIDCARRIER INCLUDING MEANS PROVIDING AN INTERNAL DUCT WITHIN SAID CARRIER(I) WHICH DUCT IS DEFINED BY OPEN ENDS AND BY SIDES FORMING A CLOSEDCROSS-SECTION AND (II) WHICH DUCT EXTENDS END-TO-END IN A DIRECTIONPARALLEL TO THE LONGITUDINAL AXIS OF SAID CHANNEL FOR THE FLOW THROUGHTHE DUCT OF THE FLUID HEAT TRANSFER MEDIUM IN A DIRECTION PARALLEL TOTHE LONGITUDINAL AXES OF THE CHANNEL AND OF THE CARRIER, AND